1. Field of the Invention
The present invention relates to a plate cylinder in a rotary press and a printing plate holder for holding a printing plate on the plate cylinder.
2. Description of the Related Art
A plate cylinder equipped with printing plates wrapped around the outer circumference thereof has been known in the art. The printing plates are fixed on the plate cylinder by means of printing plate holders. An example of such the plate cylinder is disclosed in Japanese Patent 2775850.
On the outer circumference of the plate cylinder disclosed in JP 2775850, plural attaching grooves extending parallel to the axial direction of the plate cylinder are provided in parallel with both the axial and circumferential directions. In each of these attaching grooves, a printing plate holder is provided per printing plate for holding a printing plate wrapped around the circumference of the plate cylinder.
The printing plate holder comprises a block and a printing plate engaging means. The block is fitted, secured and attached in the attaching groove. The block has a printing plate holding groove for hooking one folded end of the printing plate thereto. The printing plate engaging means is provided in the block for driving the other end of the printing plate, wrapped around the outer circumference of the plate cylinder, to be retracted always into the printing plate holding groove. The block is formed substantially in a rectangular parallelepiped shape and its top surface has a swell with the same diameter as that of the outer circumference of the plate cylinder. The printing plate holding groove is formed to have an opening along the longitudinal direction of the top surface of the block. The folded portion of the printing plate is hooked to an edge of the opening, in the longitudinal direction, at the rear in the rotational direction of the printing plate. The printing plate engaging means has a protrusion for engaging with the other end of the printing plate wrapped around the outer circumference of the plate cylinder and also has an shaft provided in the longitudinal direction of the block. The printing plate engaging means is configured to use a driving force imparted on the shaft to retract the other end of the printing plate always into the printing plate holding groove.
It is extremely difficult for JP 2775850 to process and assemble the printing plate holders provided in parallel with both the axial and circumferential directions of the plate cylinder without any substantial errors in terms of positional relations or positions of other printing plate holders relative to one printing plate holder. Accordingly, it is almost impossible to mass-produce plate cylinders that include the above two printing plate holders in position. The plate cylinder disclosed in JP 2775850 has an attaching groove formed in the outer circumference thereof for attaching a printing plate holder thereto, which is different from the plate cylinder. It is not effective, however, to correct relative positional relations of plural printing plate holders. Where overprinting such as multicolor printing is performed using a plurality of printing means equipped with such plate cylinders, although an overprinting alignment can be corrected by positioning a printing plate held by one printing plate holder on a plate cylinder in each printing means, positioning variations occur at locations of other printing plate holders. As a result, with respect to printing plates held by the other printing plate holders, overprinting errors corresponding to the above variations occur, resulting in a problem of print-failed pages.
On the other hand a technology for solving such the problem is disclosed in JP 59-31467B entitled “Split plate cylinder”, for example. In this apparatus, the outer circumference of the plate cylinder is split into two in the axial direction. Then, one plate cylinder body receives the other cylindrical member having an identical outer diameter fitted therein rotatably in the circumferential direction and movable in the axial direction. This split plate cylinder employs position-adjusting means provided each for the plate cylinder body and the cylindrical member to adjust their positions in the circumferential and axial directions to eliminate overprinting alignments. However the conventional split plate cylinder is configured to perform individual position adjusting per split section in the circumferential and axial directions and further requires rotational driving per split section. This results in problems such as a complicated structure, an increased number of components, an elevated production cost and an inefficient maintenance operation.
If plural printing plate holders are provided along the circumferential direction of the plate cylinder, the split plate cylinder disclosed in JP 59-31467B can not adjust the positions of the printing plate holders each in the circumferential direction of the plate cylinder. Accordingly, it is not possible to correct variations in relative positions on processing and assembling of the printing plate holders. In addition, it is almost impossible to mass-produce plate cylinders with matched positional relations of holding printing plates. Plural printing means equipped with the plate cylinders can be employed to perform overprinting such as multicolor printing. In this case, an overprinting misalignment can be eliminated by positioning a printing plate held by one printing plate holder on a plate cylinder in each printing means. Even though, positioning variations occur at locations of other printing plate holdings. As a result, with respect to printing plates held by the other printing plate holders, overprinting errors corresponding to the above variations occur, resulting in a problem of print failures.